Semiconductor device with improved arrangements to avoid breakage of tungsten interconnector

ABSTRACT

The object of the invention is to provide such a highly reliable semiconductor device as no defect such as the breakage of a tungsten conductor occurs. This object is achieved by the following means, i.e., a molybdenum film, a tungsten film and another molybdenum film are deposited in this order on an interlayer dielectric film formed on a silicon substrate.

BACKGROUND OF THE INVENTION

The invention relates to a semiconductor device.

In semiconductor devices, the needs for larger-integration design andfaster-speed design are high, and the micro-fabrication and faster-speeddesign of the devices have proceeded by the development of high-accuracymicromachining techniques, the enhancement of electrical characteristicsbrought about by the adoption of new materials, and the applying of newdevice structures, etc.

For interconnector-forming processes, as a material which withstandssteps performed at a temperature higher than that in the case ofaluminum alloys hitherto used and which is less apt to be broken even ina case where the width of an interconnector becomes not more than 500nanometers, tungsten (W) has come to be used as the material forinterconnectors and plugs that establish connection among theinterconnectors. Techniques for forming tungsten conductors and tungstenplugs are disclosed in JP-A-10-144623, etc.

When a tungsten film is formed at a temperature not more than 500° C. bya sputtering method or a chemical vapor deposition (CVD) method, etc.,this temperature is very low in comparison with the melting point oftungsten (about 3400° C.) and, therefore, many crystal defects such asvacancies and dislocations often are apt to remain within the grains oftungsten insofar as a period immediately after the film forming isconcerned. The vacancies and dislocations make the states of atomsunstable and provide diffusion paths within grains. For this reason,when subjected to a heat hysteresis at a temperature not less than thefilm-forming temperature, the larger the number of defects such as thevacancies and dislocations, the more the tungsten atoms tend to diffuse,and consequently, the film often becomes dense and contracts in thecourse of the diffusion of the tungsten atoms migrating to stablelocations.

Further, when a tungsten film is formed at a temperature not more thanthe above-mentioned 500° C., the grain size of tungsten often becomesabout 50 to 200 nanometers. When fine tungsten conductors each having awidth not more than 200 nano-meters are formed by performing the dryetching of the tungsten film, the tungsten conductor width and thetungsten grain size become almost equal to each other. As a result, manygrain boundaries are formed in the directions crossing the tungstenconductor, so that there occurs such a structure called “bamboostructure” as grains are present in the shape of chain. Grain boundariesare one of the locations where atoms are most apt to diffuse. Thus, theinterconnector of the bamboo structure is one of structures in which thebreaks of the tungsten conductors are most apt to occur when the atomsdiffuse actively within the tungsten conductors and when the filmcontraction occurs.

In a conventional tungsten conductor-forming step, the temperature ofheat hysteresis after forming film was set at a temperature not morethan about 500° C., and tungsten atoms were not very thermallyactivated, so that tungsten atoms did not diffuse actively. Besides,because the width of a tungsten conductor was larger than a tungstengrain size, the tungsten conductor was less apt to cause the bamboostructure. Accordingly, in the conventional interconnector-formingprocesses, the interconnectors were not broken.

However, since the width of the tungsten conductor is miniaturized to benot more than 200 nanometers, the possibility that the structure of thetungsten conductor becomes the bamboo structure is raised. Besides, in acase where a thermal load of not less than 600° C. is applied to thetungsten conductor as in the step of crystallizing amorphous tantalumoxide (Ta₂O₅) for forming a dielectric film of a capacitor or as in thestep of oxidizing the lower electrode surface of polycrystalline siliconof the capacitor, there come to occur such cases as the tungstenconductor is broken due to the diffusion of tungsten atoms caused duringsuch high-temperature steps. The smaller the width of the tungstenconductor and the higher the heat treatment temperature, the more thetungsten conductor breakage are apt to occur. It has been found that thetungsten conductor breaks are particularly remarkable when tungstenfilms are directly deposited on a silicon oxide film.

SUMMARY OF THE INVENTION

The object of the invention is to provide a semiconductor device havinghighly reliable tungsten conductors in which the above problems aresolved and defects such as no break of the tungsten conductor occurs.

The reasons for the breakage of the tungsten conductor are as follows:

(1) Because the temperature at which the film is formed is low incomparison with the melting point of tungsten, crystal defects (unstablearrangement of atoms) such as vacancies and dislocations are apt toremain in the interior of the tungsten conductor. This tendency isespecially remarkable when the tungsten interconnector is formeddirectly on the silicon oxide film.

(2) Because a heat hysteresis of a high temperature exceeding thefilm-forming temperature is applied to the sparse tungsten conductor inwhich many crystal defects remain, tungsten atoms are apt to diffusealso within grains in addition to the diffusions that occurs on thesurfaces of the tungsten conductor and that occurs at the grainboundaries.

(3) Because the tungsten conductor width is equivalent to the grain sizeof tungsten or is not more than it, the tungsten conductor comes to havethe bamboo structure, so that the tungsten conductor is broken even whenone of the tungsten grain boundaries is opened.

To solve the above problems, there is provided a semiconductor device ofthe invention that has the following features.

At least one of the above problems is solved by the followingconstitution of the invention.

(A) To suppress the surface diffusion of tungsten atoms within the abovetungsten conductor and, at the same time, to suppress the diffusionwithin the grains by lowering the proportion of the unstable arrangementof atoms remaining within the tungsten conductor, a molybdenum (Mo) film(a first electrically conductive film) and another molybdenum film (athird electrically conductive film) are formed, respectively, at theinterface defined between the tungsten conductor (a second electricallyconductive film) and an interlayer dielectric film (a first dielectriclayer), which serves as an underlayer, and on the surface side of thetungsten conductor, whereby the tungsten conductor is sandwiched betweenthe two molybdenum films.

Molybdenum has a melting point lower than that of tungsten although ithas a lattice structure similar to that of tungsten. Therefore, crystaldefects are less apt to occur in molybdenum than in tungsten. When amolybdenum film having this characteristic is used as the underlayer ofthe tungsten conductor, tungsten atoms deposit along the arrangement oftungsten atoms of the underlayer, so that a dense tungsten film havingfew crystal defects can be easily obtained. For this reason, even in acase where the tungsten conductor undergoes a heat hysteresis of atemperature not less than 500° C. at a later step, the diffusion withinthe grains or at the grain boundaries is suppressed and no breakageoccurs in the tungsten conductor.

(B) To prevent the bamboo structure of the tungsten conductor fromoccurring, a molybdenum film (a fourth electrically conductive film) isformed so that it may partition the tungsten conductor into at least twolayers (the second electrically conductive film and a fifth electricallyconductive film) in the direction of the film thickness.

The tungsten conductor comes to have a two-layer structure, so that theprobability of occurrence of tungsten conductor breakage becomes verylow even if the breakage at grain boundary occurs in the tungstenconductor of the first film, because electrical connection isestablished by the other film.

(C) To suppress the surface diffusion of tungsten atoms within thetungsten conductor, a molybdenum film (a sixth electrically conductivefilm) and another molybdenum film (a seventh electrically conductivefilm) are formed at the interface defined between the tungsten conductor(the second electrically conductive film) and the underlayer, and on thesurface side_of the tungsten conductor, respectively, whereby thetungsten conductor comes to be covered with the molybdenum films.

By making the surface, interface and side of the tungsten conductor bein contact with molybdenum having a lattice space similar to that oftungsten, the surface diffusion is suppressed and no grain-boundarybreak of a tungsten wire comes to occur.

Incidentally, in the above features of the invention (A), (B) and (C),it is not always necessary that the molybdenum films be made of puremolybdenum, that is, they may be films made of a material having alattice space similar to that of tungsten, and the use of any materialhaving the effect of suppressing the diffusion of tungsten atoms isusable. The films may be made of, for example, any one of puremolybdenum containing not less than 99% Mo by atomic ratio, a molybdenumalloy containing not less than 90% Mo by atomic ratio, molybdenumnitride containing not less than 40% Mo by atomic ratio, molybdenumcarbide containing not less than 40% Mo by atomic ratio, molybdenumboride containing not less than 40% Mo by atomic ratio, tungsten nitridecontaining not less than 40% W by atomic ratio, tungsten carbidecontaining not less than 40% W by atomic ratio, and tungsten boridecontaining not less than 40% W by atomic ratio.

The reliability against the breakage of the tungsten conductor isgreatly improved by providing a semiconductor device having the abovefeatures.

Before the embodiments of the invention are described, the followingterms used in this specification are explained. “the main elements”

The “main element” used in this specification is defined to be an“element having the highest ratio of the number of atoms in a material.”The property of this “main element” often determines the main propertyof an obtained material. “the main component”

In a compound material, when the ratio of the number of atoms of aplurality of elements, which constitute the particular compound, to thetotal number of atoms regarding a plurality of elements (includingimpurities and additives) constituting the whole of the material is thehighest, the particular compound is defined as the “main component” inthis specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a semiconductor device relatedto one embodiment of the invention.

FIG. 2 is a schematic sectional view of a first step of a method forproducing the semiconductor device related to the embodiment of theinvention.

FIG. 3 is a schematic sectional view of a second step of the method forproducing the semiconductor device related to the embodiment of theinvention.

FIG. 4 is a schematic sectional view of a third step of the method ofproducing the semiconductor device related to the embodiment of theinvention.

FIG. 5 is a schematic sectional view of a fourth step of the method ofproducing the semiconductor device related to the embodiment of theinvention.

FIG. 6 is a schematic sectional view of a fifth step of the method ofproducing the semiconductor device related to the embodiment of theinvention.

FIG. 7 is a schematic sectional view of a sixth step of the method ofproducing the semiconductor device related to the embodiment of theinvention.

FIG. 8 is a schematic sectional view of a seventh step of the method ofproducing the semiconductor device related to the embodiment of theinvention.

FIG. 9 is a perspective view of the semiconductor device encapsulatedwith a resin which semiconductor device is related to the embodiment ofthe invention.

FIG. 10 is a plan view of a portion including gate electrodes and Wconductors of the semiconductor device related to the embodiment of theinvention.

FIG. 11 is a sectional view of another semiconductor device related toanother embodiment of the invention.

FIG. 12 is a sectional view of a semiconductor device in which thestructures shown in FIG. 4 and FIG. 11 are provided.

FIG. 13 is a sectional view of a semiconductor device related to a stillanother embodiment of the invention.

FIG. 14 is a graph showing the film structure dependence of the amountof stress variation in a case where a heat treatment at 1000° C. isperformed for 10 minutes.

FIG. 15 is a graph showing the film structure dependence of the amountof specific resistance variation of a tungsten film in a case where heattreatment at 1000° C. is performed for 10 minutes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment related to the invention is shown in FIG. 1. FIG. 1 is aschematic sectional view of a semiconductor device 100 of thisembodiment in a case where the device is applied to a semiconductormemory. The left side of the figure shows the structure of a memory cellportion, and the right side of the figure shows the structure of aperipheral circuit portion.

On a silicon substrate are formed gate electrodes 4, first-layerinterconnector 16A, second-layer interconnectors 38 a, 38 b which areformed by laminating an aluminum alloy film 36 and high-melting materialfilms 35, 37, third-layer interconnectors 48 a, 48 b, 48 c which areformed by laminating an aluminum alloy film 46 and high-melting materialfilms 45, 47, a passivated film 50 for preventing humidity and the likefrom entering the interior of a semiconductor chip. On the side of thememory cell portion, a capacitor 27 is formed between the first-layerinterconnector and the second-layer interconnector.

In a case where the first-layer interconnector 16A is formed of asingle-layer tungsten film, the bamboo structure is formed when it isworked to have a width not more than 200 nanometers, so that, when athermal load not less than 600° C. is added later, tungsten atomsdiffuse and interconnector breaks at grain boundaries are apt to occur.

In the semiconductor device 100 shown in FIG. 1, the first-layerinterconnector 16A has a three-layer structure in which a tungsten film16 b is sandwiched between two molybdenum films 16 a. In this Mo/W/Molayered interconnector, the tungsten film becomes dense because of thepresence of the underlayers of the molybdenum films and, at the sametime, the surface diffusion is suppressed because both of thesubstrate-side interface and surface side of the tungsten film is incontact with the molybdenum films. Therefore, the interconnector breakat grain boundaries hardly occurs even when a thermal load not less than600° C. is applied to the first-layer interconnector 16A having ainterconnector width not more than 200 nanometers.

FIG. 2 to FIG. 8 are schematic sectional views of the steps of producingthe semiconductor memory relating to the embodiment. FIG. 2schematically shows a section of the device at the stage of forming acontact hole 10 and a through hole 11 for obtaining electricalcontinuity after the steps of forming of an MOS(metal-oxide-semiconductor) transistor on a silicon substrate 1 anddepositing interlayer dielectric films 6,9.

Then, shallow trenches 3 for separating elements are formed in thesilicon substrate 1, a silicon oxide film 2 being formed on the surfaceof the trenches, a silicon oxide film 3 a being embedded in the shallowtrenches 3, a gate oxide film 2 a being formed, gate electrodes 4 and asilicon nitride film 5 covering the gate electrodes 4 being formed,impurities being added into the interior of the silicon substrate 1, theinterlayer dielectric film 6 being formed, polycrystalline silicon plugs8 a, 8 b being embedded in the contact holes 7, the interlayerdielectric film 9 being formed, and the contact holes 10 and throughholes 11 are formed.

In FIG. 3, to prevent the contamination of the silicon substrate byheavy metals coming from the interconnectors and the like, high-meltingpoint material films 12 used as barrier films are formed within thecontact holes 10 and the through holes 11. The high-melting pointmaterial films 12 are formed by depositing and laminating, for example,a titanium (Ti) film with a thickness of 10 nm and a titanium nitride(TiN) film with a thickness of 100 nm by use of a sputtering method or aCVD method. At the interfaces each defined between the high-meltingpoint material film 12, the silicon substrate 1 and the polycrystallinesilicon plug 8b are formed silicide layers 15 brought about by achemical reaction in a heat treatment step that is added later. Forexample, titanium silicide layers are formed when the high-melting pointmaterial film 12 has a layered structure of a Ti film and a TiN film,and cobalt silicide layers are formed when the high-melting pointmaterial film 12 has a layered structure of a cobalt film and a TiNfilm.

After forming the high-melting point material film 12, a tungsten film13 is deposited by a chemical vapor deposition (CVD) method, and thetungsten film 13 is embedded in the contact holes to thereby form atungsten plug 14. The high-melting point material film 12 and tungstenfilm 13 deposited on the interlayer dielectric film 9 are polished andremoved by a chemical mechanical polishing (CMP) method with thetungsten plug 14 alone kept remaining, and the surface of the interlayerdielectric film 9 is flattened.

In FIG. 4, on the interlayer dielectric film 9, the molybdenum film 16a, the tungsten film 16 b and the molybdenum film 16 a are formedthrough a sputtering method or a CVD method. The thicknesses of thedeposited films are such that, for example, the molybdenum film 16 a is10 nm, the tungsten film 16 b is 100 nm and the molybdenum film 16 a is10 nm. Because the resistance of a molybdenum film is as low as that ofa tungsten film, the Mo/W/Mo layered structure is superior in electricalproperties to a TiN/W/TiN structure in which each of the titaniumnitride (TiN) films is formed at the interface defined by each of theinterlayer dielectric films. Therefore, the smaller the interconnectorwidth, the more this structure is effective.

The methods of depositing the tungsten and molybdenum films aredescribed below. In the sputtering method, atoms are physicallysputtered from a target by using the cations of a noble gase such asargon and are deposited on a wafer. Therefore, this method has theadvantages of excellent adhesion to an underlayer and a highfilm-forming rate. Conversely, when a film is to be deposited in theinterior of a deep trench, the thickness of a film adhering to the sideof the trench becomes smaller than that of a film adhering to the bottomof the trench, thus causing a disadvantage. On the other hand, when afilm is deposited by the CVD method, the film relatively uniformlyadheres to both the side and bottom of the trench although the adhesionto the underlayer tends to be weak in comparison with the sputteringmethod. Therefore, the CVD method is suitable for a case where a film isembedded in the interior of a deep trench or a hole. Both methods havetheir advantages and disadvantages and, therefore, films can bedeposited by making use of the advantages while taking device structuresand film stresses etc. into consideration.

As regards a film-forming temperature, the higher the film formingtemperature, the denser the tungsten film 16 b becomes, and the lowerelectrical resistance becomes, with the result that the tungsten film 16b can be used as narrower interconnectors. Incidentally, at thisfilm-forming stage, the first-layer interconnector 16A may be beforehandsubjected, in a vacuum, to a heat treatment of the same temperature as aheat hysteresis to be added layer, whereby the Mo/W/Mo layeredinterconnectors become denser and the interconnectors become less apt tobe broken.

As regards a film-forming rate, in a case where a tungsten film-formingrate decreases, individual tungsten atoms that fly onto a wafer canmigrate to more stable locations, so that the tungsten becomes denserand, therefore, interconnectors become less apt to be broken.

Next, the layered interconnector 16A having a width not more than 0.2 μmis formed by the dry etching of the Mo/W/Mo layered film. The surface ofthe interlayer dielectric film 9, which is the underlayer of thefirst-layer interconnector 16A, is polished and planated by the CMP and,therefore, this permits the exposure of a high-accuracy and fineinterconnector circuit.

A plan arrangement of the device is explained below by referring to FIG.10. In the figure, the layers extending in a vertical direction are thegate electrodes 4, and each of the W conductors 16A is located in thespace defined between the polycrystalline silicon plugs 8 a and extendsin a direction vertical to the gate electrodes. The sectional viewsdisclosed above are ones obtained by taking cross sections along theline A—A in the plan view. Each of the transistors is formed in a region1 a formed like an island on the surface of the silicon substrate 1, andeach of the shallow trenches 3 around the regions la is embedded withthe silicon oxide film 3 a. Electrical signals from this regions 1 a ineach of which the transistor is formed are taken out through thepolycrystalline silicon plugs 8 a, 8 b. Onto each of the polycrystallinesilicon plugs 8 b is further connected a tungsten plug 14, which is inturn connected to the tungsten conductor. Each of capacitors 27 isconnected to the polycrystalline silicon plug 8 a.

FIG. 14 shows examples of variations of residual stresses occurring inthe film in cases where a sample obtained by sputtering a single-layertungsten film on a silicon oxide film and another sample obtained bydepositing an Mo/W/Mo layered film were subjected to a heat treatment at1000° C. The thickness of each of the films deposited on each of thesamples was such that the tungsten film was 100 nm, and in the Mo/W/Molayered film the molybdenum film was 5 nm, the tungsten film was 95 nm,and the molybdenum film was 5 nm. Incidentally, it was ascertained that,because the molybdenum film was as thin as 5 nm, the stress variation ofthe single layer of molybdenum before and after the heat treatment wasvery small in comparison with the stress variation of the whole Mo/W/Mostructure.

Before the forming of the films, large compressive stresses of about 5GPa occurred in both structures, and the measured samples were warpedinto a convex shape with their film-deposited sides facing upward.However, after the heat treatment at 1000° C., the film stress in thestructure of the single tungsten layer decreased to about one-tenth ofthe initial stress value. It is thought that the film was contractedabruptly, resulting in the generation of a stress variationcorresponding to a tensile stress of about 4 GPa. In the Mo/W/Mostructure, the stress decreases down to about 2.7 GPa due to the heattreatment at 1000° C., however, it is found that the stress variationbecomes very small in comparison with the structure of single tungstenlayer.

This shows that the larger the variation of the film stress, the sparserthe original film, with the result that during heat treatment, the filmbecomes dense and contracts. Conversely, in the Mo/W/Mo structure, thefilm is thought to become denser than the tungsten film of the singlelayer because of the effect of the underlayer of the molybdenum film.From the respects, it is thought that the film of the single tungstenlayer is sparser and that in the Mo/W/Mo layered film structure, the twoupper and lower molybdenum layers provide the effects of the underlayerand of suppressing surface diffusion.

For the above structure of the single tungsten layer and the Mo/W/Molayered structure, FIG. 15 shows examples of variation of the specificresistance before and after heat treatment. In the figure, the ordinateindicates the specific resistance. In the sputtered single-layertungsten film, the specific resistance is not less than 100×10⁻⁶ (Ω/m)and this value is not less than 20 times higher than the specificresistance of the tungsten balk, which is 4.9×10⁻⁶ (Ω/m). It is foundthat the property of the film becomes such that currents are less apt toflow because of many crystal defects and dislocations included therein.In the case of the Mo/W/Mo layered structure, the specific resistance isabout 40×10⁻⁶ (Ω/m), which is not more than 40% of the specificresistance of the single-layer tungsten film, and it is found that thecrystallizability of the tungsten film is improved by the presence ofthe molybdenum layers used as the underlayer. When comparing thespecific resistance before and after the heat treatment at 1000° C., thespecific resistance of the Mo/W/Mo layered film decreases about 30%after the heat treatment, whereas in the case of the single-layertungsten film the specific resistance decreases greatly down to one-halfor more. Similarly to the results of the variations of the film stressesshown in FIG. 14, from the changes in specific resistance, it is alsofound that the single-layer tungsten film is sparse, whereas the Mo/W/Molayered film is denser and hence is less apt to change when subjected toheat treatment.

Other structures of the first-layer interconnector made of a materialcontaining tungsten are described by referring to FIG. 11 to FIG. 13.

In FIG. 11, the tungsten film 16 b has a W/Mo/W three-layer structure16B in which a thin molybdenum film 16 c is interposed in the middle ofthe tungsten film 16 b so that the tungsten film may be divided into twolayers. Each layer is deposited by the sputtering method or the CVDmethod, and the layer thicknesses are such that, for example, thelower-layer of the tungsten film 16 b is 40 nm, the middle molybdenumlayer 16 c being 10 nm, and the upper-layer tungsten layer 16 b is 60nm. Because of the presence of the thin molybdenum layer 16 c fordividing the tungsten film in the middle thereof, the tungsten grainswithin the first-layer interconnector 16B are divided into at least twoupper and lower portions of the interconnector and this layeredstructure becomes different from the bamboo structure. In this layeredstructure, therefore, even when interconnector break at grain-boundariesoccurs in either one of the upper and lower layers, this does not leadto any break of the interconnector as a whole. Thus, this layeredstructure comes to have a high reliability.

In FIG. 12, the first-layer interconnector has such a structure 16C as aMo/W/Mo layered structure in which a tungsten layer 16 b is sandwichedbetween two molybdenum layers 16 a has a thin molybdenum layer 16 c fordividing the tungsten layer 16 b in the direction of the film thickness.Therefore, the interconnector structure 16C has a higher reliabilitythan in FIG. 4 and FIG. 11 because of the effect of the dense tungstenlayer 16 b brought about from the presence of the molybdenum layer 16 aused as the underlayer, the surface diffusion suppressing effect broughtabout from the contact of the upper and lower interfaces of the tungstenlayer 16 b with the molybdenum layer 16 c, and the effect of thedivision of the tungsten layer 16 b into the upper and lower layers.

Still another interconnector structure 16D is described below whilereferring to FIG. 13. In this structure 16D, a tungsten layer 16 bbecomes dense because a molybdenum layer 16 d is formed at the interfaceof the underlayer of the tungsten film 16 b, and surface diffusion ismost suppressed because a molybdenum layer 16 d and another molybdenumlayer 16 e are formed at the interfaces surrounding the tungsten layer16 b.

The manufacturing process of the interconnector structure 16D isdescribed below. Trenches are first formed in an interlayer dielectricfilm 9 b, a thin molybdenum layer 16 d being then deposited in concaveshape along the interior of each of the trenches, and the tungsten layer16 b is deposited so that the concave portions of the molybdenum layer16 d are embedded with the tungsten layer 16 b. The thicknesses of thedeposited layers are such that, for example, the molybdenum layer 16 dis 10 nm and the tungsten layer is 200 nm. Next, the tungsten layer 16 band molybdenum layer 16 d that are deposited in portions other than theinteriors of the interlayer dielectric film 9 are polished and removedby CMP, whereby a structure is formed in which the molybdenum layer 16 dand tungsten layer 16 b are embedded in the interior of each of thetrenches formed in the interlayer dielectric film 9. Finally, themolybdenum layer 16 e is deposited and etching is performed in a mannerthat only the surface portions of the molybdenum layer 16 d and tungstenlayer 16 b are made to remain, whereby the interconnector structure isformed in which, as shown in FIG. 13, the tungsten layer 16 b is coveredwith the molybdenum layers 16 d and 16 e.

Next, the step of FIG. 5 that follows the step of FIG. 4 is describedbelow. A interlayer dielectric film 17 is formed on the Mo/W/Mo layeredinterconnector 16A. When the interlayer dielectric film 17 is made to bea layered structure, a layer-forming step is repeated by the same timesas the number of the laminated layers. A through hole 20 for a capacitorof the interlayer dielectric film 17 is formed so that it may bedisposed in the middle of the Mo/W/Mo layered interconnector 16A.

In FIG. 6, a polycrystalline silicon film is embedded in the formedthrough hole 20, the polishing and removal of the polycrystallinesilicon film on the interlayer dielectric film 17 and the planarizationof the surface of the interlayer dielectric film 17 are performedthrough the CPM process, whereby a polycrystalline silicon plug 21 isformed. Next, an interlayer dielectric film 22 is formed, trenches 23for a capacitor being formed by dry etching, and a polycrystallinesilicon film 24, which serves as lower electrodes of the capacitor, isformed along the interior of each of the trenches for a capacitor.

In this case, the lower electrodes may be formed of an electricallyconductive material other than the poly-crystalline silicon film 24, andhigh-melting metals, such as platinum and ruthenium, and electricallyconductive metal compounds, such as TiN, tantalum nitride (TaN),ruthenium oxide (RuO) and iridium oxide (IrO), may be used because theysuffer very little degradation in heat resistance and oxidationresistance even when high-temperature treatment is performed after thesucceeding forming of a dielectric film for a capacitor.

In FIG. 7, a polycrystalline silicon film 24 a on the interlayerdielectric film 22 is first polished by a CMP or the polycrystallinesilicon film 24 a is subjected to dry etching by embedding a resist inthe trench 23 for a capacitor and the resist is removed by ashing, etc.,whereby the lower electrodes 24 made of polycrystalline silicon isseparated in cup shape. Next, a tantalum oxide (Ta₂O₅) film 25 a isdeposited in an amorphous state with a thickness of 20 nm and theamorphous Ta₂O₅ is crystallized by heat treatment at 700° C. to form apolycrystalline Ta₂O₅ film 25 b.

Although this step of heat treatment at 700° C. is indispensable forgiving the polycrystalline Ta₂O₅ film 25 b a dielectric constant thatmeets product specifications, a large thermal load is applied, duringthis step, to the first-layer interconnector 16A formed beforehand. Whenthe first-layer interconnector 16A is a single tungsten layer, tungstenatoms diffuse and tungsten grain boundaries are opened, with the resultthat the interconnector break becomes apt to occur in the tungstenconductor. However, the break of the first-layer interconnector 16A isprevented by making the first-layer interconnector 16A have thethree-layer structure of molybdenum layer 16 a, tungsten layer 16 b andmolybdenum layer 16 a. Therefore, because no interconnector break occurseven when the first-layer interconnector 16A is worked to a narrowerinterconnector with a width not more than 200 nm, it is possible toprovide a high-reliability semiconductor device 100. Furthermore, a costreduction by an improvement in the yield of the semiconductor device 100can be expected.

In the above embodiment, Ta₂O₅ is used as the material for thedielectric film of the capacitor. However, because the object of theinvention is to provide a highly integrated semiconductor device, thematerial for the dielectric film is not limited to Ta₂O₅, and materialshaving a larger dielectric factor than that of silicon oxide can beused. The dielectric film of the capacitor may be formed of a materialselected, as the main component, for example, from silicon nitride(Si₃N₄), titanium oxide (TiO₂), strontium- bismuth-tantalum oxide(SrBi₂Ta₂O₉: SBT), strontium titanate (SiTiO₃: STO), bariumtitanate-strontium ((Ba_(x)Sr_(1−x))TiO3: BST), and lead zirconatetitanate (Pb(Zr_(x)TiO_(1−x))O3: PZT). In the above materials, the finalatomic ratio may sometimes differ depending on a difference in theprocess of forming the films. However, these materials may be usedinsofar as the dielectric constant thereof not less than 5 is concerned.

Moreover, it is not necessary that the high temperature step in which atemperature not less than 600° C. is used be the step of forming thedielectric film, and other processes in which a temperature not lessthan 600° C. is used to form other films may be included.

In FIG. 8, a TiN film that serves as the upper electrodes is formed bythe CVD method so that it is uniformly deposited along the interior ofthe capacitor, and a circuit is formed by dry etching. On the surface ofthe formed upper electrode 26 is formed an interlayer dielectric film30, and a through hole 31 is formed for providing electric continuity tothe circuit surrounding the substrate.

After performing the steps up to the state shown in FIG. 8, there arefurther performed the steps of: forming a tungsten plug 34 for theelectrical connection between the first-layer interconnector 16A andsecond-layer laminated interconnectors; forming a second-layer layeredinterconnectors 38 a, 38 b; forming an interlayer dielectric film 40;forming tungsten plugs 44 a, 44 b for the electrical connection betweenthe second-layer interconnectors and third-layer interconnectors andbetween the upper electrode 26 of the capacitor and the third-layerinterconnectors forming the third-layer interconnectors 48 a, 48 b,forming a silicon oxide film 49 and a silicon nitride film 50 thatprotect the whole of the semiconductor device 100; and forming openings(not shown in the drawings) for the electrical connection to the outsideof the semiconductor device 100, whereby the semiconductor device 100having high reliability is completed even in a case where the finefirst-layer interconnector 16A with a width not more than 200 nm isformed on the side of the substrate of the capacitor as shown in FIG. 1.

Finally, packaging in a chip size or packaging in a shape as shown inFIG. 9 is applied. FIG. 9 shows an example in which the semiconductordevice 100 is encapsulated with a resin 101. The semiconductor device100 is bonded onto a die electrode, and a bonding wire 103 is connectedto the semiconductor device 100. The bonding wire 103 is also connectedto a lead frame 104 and performs the input and output of signals to theoutside.

Because of these features, the invention can provide a highly reliablesemiconductor.

Further, according to the invention, defects such as the break oflayered interconnectors can be prevented and the reliability againstinterconnector breaks in a semiconductor device is greatly improved.

What is claimed is:
 1. A semiconductor device comprising: a siliconsubstrate; an interlayer dielectric film formed on said siliconsubstrate; and a layered interconnector having a first electricallyconductive film formed on a surface of said interlayer dielectric filmwhich first conductive film contains molybdenum as the main elementthereof, a second electrically conductive film formed on a surface ofsaid first electrically conductive film which second conductive filmcontains tungsten as the main element thereof, and a third electricallyconductive film formed on a surface of said second film which thirdconductive film contains molybdenum as the main element thereof.
 2. Asemiconductor device according to claim 1, wherein the layeredinterconnector has a width not more than 200 nanometers.
 3. Asemiconductor device comprising: a silicon substrate; a first dielectricfilm formed on said silicon substrate; a layered interconnector having afirst electrically conductive film formed on a surface of saidinterlayer dielectric film which first conductive film containsmolybdenum as the main element thereof, a second electrically conductivefilm formed on a surface of said first electrically conductive filmwhich second conductive film contains tungsten as the main elementthereof, and a third electrically conductive film formed on a surface ofsaid second film which third conductive film contains molybdenum as themain element thereof; a second dielectric film deposited on a surface ofsaid layered interconnector; and a dielectric film for a capacitativeelement which dielectric film is deposited at a side of said seconddielectric film which side is opposite to the layered interconnector. 4.A semiconductor device comprising: a silicon substrate; an interlayerdielectric film formed on said silicon substrate; and a layeredinterconnector having a second electrically conductive film formed on asurface of said interlayer dielectric film which second conductive filmcontains tungsten as the main element thereof, a fourth electricallyconductive film formed on a surface of said second electricallyconductive film which fourth conductive film contains molybdenum as themain element thereof, and a fifth electrically conductive film formed ona surface of said fourth conductive film which third conductive filmcontains tungsten as the main element thereof.
 5. A semiconductor devicecomprising: a silicon substrate; a first dielectric film formed on saidsilicon substrate; a layered interconnector having a second electricallyconductive film formed on said interlayer dielectric film which secondconductive film contains tungsten as the main element thereof, a fourthelectrically conductive film formed on said second electricallyconductive film which second conductive film contains molybdenum as themain element thereof, and a fifth electrically conductive film formed onsaid fourth conductive film which fifth conductive film containstungsten as the main element thereof; a second dielectric film formed ona surface of said layered interconnector; and a dielectric film for acapacitative element which dielectric film is formed at a side of saidsecond dielectric film which side is opposite to the layeredinterconnector.
 6. A semiconductor device comprising: a siliconsubstrate; a first dielectric film formed on said silicon substrate;trenches each formed in a surface of said first dielectric film; and alayered interconnector having a sixth electrically conductive filmformed in a concave shape along an inner surface of each of saidtrenches which sixth conductive film contains molybdenum as the mainelement thereof, a second electrically conductive film formed on aconcave surface of said sixth electrically conductive film which secondconductive film contains tungsten as the main element thereof, and aseventh electrically conductive film formed on a surface of said secondconductive film which seventh conductive film contains molybdenum as themain element thereof.
 7. A semiconductor device comprising: a siliconsubstrate; a second dielectric film formed on said silicon substratewhich dielectric film is provided with trenches each formed in a surfaceof said first dielectric film; and a layered interconnector having asecond electrically conductive film formed in an inside of each of saidtrenches which second conductive film contains tungsten as the mainelement thereof, a sixth electrically conductive film formed at aninterface defined between said first dielectric film and said secondconductive film which second conductive film contains molybdenum as themain element thereof, and a third electrically conductive film formed ona surface of said second conductive film.
 8. A semiconductor devicecomprising: a substrate made of silicon; a first dielectric filmdeposited on said silicon substrate which first dielectric film hastrenches formed in a surface of the dielectric film; a layeredinterconnector made of a material containing tungsten, said layeredinterconnector being provided with a second electrically conductive thinfilm formed at the inside of each of said trenches which secondconductive film contains tungsten as the main element thereof, a sixthelectrically conductive thin film formed at an interface defined betweensaid first dielectric film and said second conductive film which sixthconductive film contains molybdenum as the main element thereof, aseventh electrically conductive film formed on a surface of said secondconductive film so that said seventh electrically conductive film is incontact with both of said second conductive film and said sixthconductive film; a second dielectric film formed on a surface of saidlayered interconnector; and a dielectric film for a capacitative elementwhich dielectric film is deposited on a surface side of said seconddielectric film.
 9. A semiconductor device comprising: a siliconsubstrate; an interlayer dielectric film formed on said siliconsubstrate; and an electrically conductive interconnector layer providedadjacently to said interlayer dielectric film, said electricallyconductive interconnector layer having a layered structure provided witha first electrically conductive film containing tungsten as the mainelement thereof, and a second electrically conductive film continuingmolybdenum as the main element thereof which second electricallyconductive film is located between said first electrically conductivefilm and said interlayer dielectric film.